The invention relates to an arrangement for compensating the axial thrust of a fluid-flow machine, having a load-relieving element, which is connected for conjoint rotation to a shaft and, together with a counter-element fixed in relation to the casing, forms a radial flow-restrictor gap.
Axial thrust is the resultant of all the axial forces acting on the rotor of a fluid-flow machine. A distinction is drawn between different types of axial thrust compensation.
Essentially three types of load-relieving devices are known for absorbing the axial thrust: balancing disk, single-acting piston and double-acting piston. Common to all three embodiments is a relief flow guided through gaps. The relief flow, which is generally fed hack to the inlet of the centrifugal pump, represents a leakage loss, with gap widths that are as small as possible being used in an attempt to minimize this.
The aim is to achieve a controlled axial position of the rotor in all operating states of the fluid-flow machine in order to ensure trouble-free operation of the fluid-flow machine. During the operation of the centrifugal pump, rubbing of moving parts against fixed parts must be avoided.
During the operation of a fluid-flow machine with a balancing disk, the pressure difference acting between two sides of the load-relieving element leads to a load-relieving force which is opposed to the axial thrust. Here, the load-relieving force is precisely equal to the axial thrust. There is an equilibrium of forces on the rotor. Rubbing of the load-relieving element against the counter-element is prevented.
During startup and shutdown processes, this pressure difference has not yet been built up, with the result that contact occurs between the load-relieving element and the counter-element without appropriate countermeasures. Such cases of the radial gap surfaces running together is supposed to be prevented with the aid of spring assemblies in the “lift-off device”.
German patent document no. DE 886 250 describes a lift-off device for centrifugal pumps. The lift-off device forms a separate component element, the rotating parts of which are secured on the shaft stub of the centrifugal machine, said stub facing away from the input side. The nonrotating parts are supported on the casing of the centrifugal machine. In these conventional devices for absorbing the axial forces during startup and shutdown, spring assemblies are arranged in a separate space outside the region through which fluid flows. This space is sealed off from the pumping medium. Such designs lead to an extended overall length. Moreover, a dedicated casing must be provided for the device.
Another possibility for taking up the axial forces during startup and shutdown processes of fluid-flow machines consists in the use of a Cardan ring.
German patent document no. DE 199 27 135 A1 describes a load-relieving device for multi-stage centrifugal pumps in which a Cardan ring is used. The Cardan ring is dimensioned in such a way that it is elastically deformed by the residual thrust. The Cardan ring is arranged in a separate sealed space. This design too leads to an additional overall length of the fluid-flow machine.
A device for limiting the axial thrust of the pump rotor of a centrifugal machine is described in German patent document no. DE 1 745 898 U. The load-relieving element is prevented from running up against the counter-element by the fact that a freely movable thrust bearing and an outer bearing ring rest against a support bearing flange owing to spring force, exercising a limiting effect. Oil lubrication is required for this design. With this design too, the overall length of the machine is increased by a corresponding shaft section where the corresponding casing is sealed off from the pumping medium.
It is the object of the invention to specify an arrangement for compensating the axial thrust of a fluid-flow machine, in which the load-relieving element is reliably prevented from rubbing against a counter-element, even during startup or shutdown. At the same time, prevention of running contact should not lead to an additional overall length of the fluid-flow machine. A dedicated casing and the use of an additional lubricant should also be avoided.
According to the invention, this object is achieved by virtue of the fact that a device for maintaining the distance between the load-relieving element and the counter-element is arranged on the counter-element, said device having at least one force-generating element, which generates a force opposed to the axial thrust.
According to the invention, the device is arranged on the counter-element itself. The device is situated in the region of the fluid-flow machine through which the medium flows. Thus, there is no need for any extension of the shaft or an additional casing. Moreover, a separate lubricant is eliminated with the device according to the invention.
The device has at least one force-generating element, which generates a force opposed to the axial thrust. The force-generating element can operate hydraulically or magnetically, for example. The use of piezoelectric elements for force generation is also possible.
In a particularly advantageous embodiment of the invention, a spring is used as the force-generating element. This is inexpensive to produce and proves to be extremely reliable in preventing the load-relieving element from rubbing against the counter-element. Moreover, no additional driving means are required.
The device preferably has an axially movable element in addition to the force-generating element. In an advantageous embodiment of the invention, the axially movable element has at least one region which engages at least partially in a guide formed by the counter-element. For this purpose, the axially movable element can have an annular projection which engages in an annular recess in the counter-element.
In a preferred embodiment of the invention, the force-generating element is arranged between the counter-element fixed in relation to the casing and the axially movable element. In this case, a space is formed in which the force-generating element is positioned. Here, the force-generating element can be supported on the counter-element and act on the axially movable element.
The load-relieving element has surfaces which face a high-pressure space and surfaces which face a low-pressure space. During the operation of the fluid-flow machine, the pressure difference between the high-pressure space and the low-pressure space leads to a load-relieving force which is opposed to the axial thrust. Here, the load-relieving force is precisely equal to the axial thrust. There is an equilibrium of forces on the rotor. Rubbing of the load-relieving element against the counter-element is prevented.
Since this pressure difference has not yet been built up during startup or shutdown processes, provision is made, according to the invention, for a force to be built up by the force-generating element during startup and/or shutdown, said element being arranged on the counter-element. This force acts on an axially movable element. As a result, the axially movable element is moved in the direction of the load-relieving element.
A sliding bearing element is preferably arranged on the axially movable element. In a particularly advantageous embodiment of the invention, the sliding bearing element is composed of a high-strength thermoplastic. A plastic based on polyaryletherketones has proven particularly advantageous here. Polyetheretherketone (PEEK) is preferably used. This sliding element allows sliding lubricated by the medium.
During startup or shutdown processes, the axially movable element is moved to such an extent by the force-generating element that the sliding bearing element rests on the load-relieving element. The sliding bearing element thus serves as a stop for the load-relieving element, thus preventing the load-relieving element from rubbing against the counter-element.
By virtue of the choice of material according to the invention, sliding of the load-relieving element on the sliding bearing element is lubricated by the medium, preventing the load-relieving element from rubbing against the counter-element. Damage to the counter-element and/or the load-relieving element is thereby prevented. No abrasion occurs, and therefore the desired geometry of the radial flow-restrictor gap is maintained.
The sliding bearing element is preferably a ring. In a particularly advantageous embodiment of the invention, the ring is arranged in a receptacle formed by the axially movable element. For this purpose, the axially movable element can have a groove, in which the sliding element rests. The sliding element is preferably fixed on the axially movable element by means of an adhesive and/or of some other fixing means.
During the startup phase, the axially movable element is moved toward the load-relieving element. If a pressure difference then builds up between the high-pressure space and the low-pressure space, a pressure difference acts on the load-relieving element, leading to a load-relieving force and moving the rotor counter to the axial thrust.
When the fluid-flow machine is running at the desired speed, a pressure acts on the axially movable element and the sliding bearing element. As a result, the axially movable element moves away from the load-relieving element toward the counter-element. The axially movable element, together with the sliding bearing element, is thus moved into a retracted position.
A sealing element is preferably arranged between the axially movable element and the counter-element. Using this sealing element, a high-pressure space is separated from a low-pressure space. The force-generating element is arranged in a space in which lower pressure prevails.
The counter-element preferably has an opening, which connects the space in which the force-generating element is arranged to the load-relieving space. With this connection, medium can escape from or flow into the space when the axially movable element is moved.
As the machine is shut down, e pressure difference between the high-pressure and the low-pressure space decreases, with the result that the load-relieving force declines and the flow-restrictor gap becomes smaller. As a result, the pressure acting in the high-pressure space on the axially movable element and the sliding bearing element also decreases. Thus, a new equilibrium is established, at which the force-generating element moves the axially movable element toward the load-relieving element. As a result, the axially movable element moves forward. In this position, the balancing disk rests against the sliding bearing element and prevents the load-relieving element from rubbing against the counter-element.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.